Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into hydrocarbons.
This second step, the preparation of hydrocarbons from synthesis gas, is well known in the art and is usually referred to as Fischer-Tropsch synthesis, the Fischer-Tropsch process, or Fischer-Tropsch reaction(s). Fischer-Tropsch synthesis generally entails contacting a stream of synthesis gas with a catalyst under temperature and pressure conditions that allow the synthesis gas to react and form hydrocarbons.
More specifically, the Fischer-Tropsch reaction is the catalytic hydrogenation of carbon monoxide to produce any of a variety of ranging from methane to higher alkanes, olefins and oxygenated hydrocarbons, including aliphatic alcohols. Research continues on the development of more efficient Fischer-Tropsch catalyst systems and reaction systems that increase the selectivity for high-value hydrocarbons in the Fischer-Tropsch product stream.
There are continuing efforts to design reactors that are more effective at producing these desired products. Product distribution, product selectivity, and reactor productivity depend heavily on the type and structure of the catalyst and on the reactor type and operating conditions. It is particularly desirable to maximize the production of high-value liquid hydrocarbons, such as hydrocarbons with five or more carbon atoms per hydrocarbon chain (C5+). Components (C5+) that boil at temperatures above 40° C., are herein defined as “heavy components.” “Light components” are defined as materials that do not condense at 0° C. or higher.
Originally, the Fischer-Tropsch synthesis was operated in packed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated multiphase reactors sometimes called “slurry reactors” or “slurry bubble columns,” operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are converted to gaseous and liquid products. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor. The liquid products are recovered from the suspending liquid by a variety of techniques such as settling, filtration, magnetic separation techniques, hydrocyclones, etc.
Gas-agitated multiphase reactors inherently have very high heat transfer rates and, therefore, reduced reactor cost. This and the ability to remove and add catalyst online are some of the main advantages of such reactors as applied to the Fischer-Tropsch synthesis, which is exothermic.
Sie and Krishna (Appl. Catalysis A: General, (1999) 186 55–70) give a history of the development of various Fischer Tropsch reactors and the advantages of slurry bubble columns over fixed bed reactors.
Much previous work has been aimed at optimization of the slurry bubble column system for Fischer Tropsch and other chemistries. Wu and Gidaspow, (Chem. Eng. Sci, 2000, 55, 573–587) show examples of numerical simulations of hydrodynamics of Slurry Bubble Column processes. Letzel, Schouten, Krishna and van den Bleek (Chem. Eng. Sci, 1999, 54, 2237–2246) developed a simple model for gas holdup and mass transfer at high pressure in a slurry bubble column. Maretto and Krishna (Catalysis Today (1999), 52, 279–289), developed a two bubble class model that could be used to identify parameters for the increase in the reactor productivity. Sanyel, Vasquez, Roy, and Dudukovic (Chem. Eng. Sci. (1999), 54, 5071) and Pan, Dudukovic, and Chang (Chem. Eng. Sci., (1999), 54, 2481) showed examples of computational fluid dynamic modeling and optimization of a slurry bubble column reactor irrespective of the chemistry. Krishna, DeSwart, Ellenberger, Martina, and Maretto (AIChE J. 1997, 43(2) 311) measured experimentally the increase in gas holdup with an increase in the gas velocity and solid concentration in a slurry bubble column in churn turbulent regime.
Considerable patent literature addresses the optimization of the Fischer Tropsch Slurry Bubble Column reactor (SBCR) and the overall system. U.S. Pat. No. 5,348,982 describes a mode of operation for SBCR. U.S. Pat. No. 6,060,524 and U.S. Pat. No. 5,961,933 show that improved operation can be obtained by introduction of liquid recirculation.
Van der Laan, Beenackers, and Krishna (Chem. Eng. Sci. (1999), 54, 5013) showed that for a Fischer Tropsch SBCR using an iron catalyst that the reactor conversion will decrease and productivity will increase with increases in the inlet superficial gas velocity. Jackson, Torczynski, Shollenberger, O'Hem, and Adkins (Proc. Annual Int. Pittsburgh Coal Conf. (1996), 13th (Vol 2) 1226) showed experimental evidence of the increase of gas hold up with increase in the inlet superficial velocity in a SBCR for Fischer Tropsch synthesis. Saxena (Cat. Rev. -Sci. Eng. (1995) 37, 227) also presents detailed experimental findings and correlations for the optimization of a Fischer Tropsch SBCR. It is clear from all the work in industry and academia that there is a need for an optimized Fischer Tropsch reactor and reactor configuration.
Current commercial slurry reactors for the Fischer-Tropsch process are envisioned in Choi, Kramer, Tam, and Fox (1996, paper presented at the 1997 Spring AIChE meeting) as first and second stage slurry bubble columns where the number of reactors in the first stage is twice the number in the second stage. An example of this configuration includes using four slurry bubble reactors in parallel as a first stage and another two parallel reactors in a second stage. Between the two stages a condenser is typically used to condense the heavy components, and in some operations water, from the gas stream. High conversion rates are achieved from this process. However, the costs associated with this design are high, due to the number and size of reactors required. It is desirable to design a gas-agitated multiphase reactor system that enables maximum reactor productivity or minimized reactor volume.
The performance of a SBCR is a combined result of reaction kinetics, heat and mass transfer, and multiphase hydrodynamics. At a given reactor geometry and operating conditions, the productivity of an SBCR is related to the catalyst activity. For a given catalyst, and therefore a given activity, the reactor productivity changes with the gas flow rate at the reactor inlet. As illustrated in FIG. 1, a high gas flow rate corresponds to a high gas holdup in the reactor. Increased gas holdup would reduce the slurry volume in the reactor. The conversion also decreases with linear velocity in other reactors like fixed bed ones, as a result of contact time, as well as reduced liquid volume as a result of increased gas holdup. Therefore the conversion in the reactor decreases with an increase in superficial inlet gas velocity even faster than what would be expected if the liquid volume remained constant, as shown in FIG. 2. On the other hand, the high gas flow rate gives a large gas-liquid contact area, contributing to a high productivity, as shown in FIG. 3. An optimum solution can be found based on the facts that the conversion decreases and the productivity increases with the increase of inlet gas flow rate. Previous work has not examined closely the interaction between reactor variables and the way in which the overall reactor system is configured.